In a typical direct melt forming operation, glass batch materials are heated in a large melting furnace and fed via a refiner to an elongated refractory-lined forehearth having a plurality of bushing assemblies connected to apertures in the bottom of the forehearth. Glass fibers are formed by attenuating streams of molten glass through orifices or nozzles in the bottom tip plate of each bushing assembly.
In conventional bushings, the flow pattern through the bushing is generally determined by the configuration, number and positioning of the orifices. The bushing throughput can be affected by fluctuations in the temperature of the bushing tip plate and molten glass contained in the bushing reservoir. To control bushing throughput, U.S. Pat. No. 4,436,541 discloses a method and apparatus for forming mineral fibers in which an orificed member is positioned above the bushing tip plate and extends between the sidewalls of the bushing (col. 1, lines 38-45). In an example at col. 2, lines 60-62, the orificed member has uniformly distributed 0.039 inch diameter orifices on 0.25 inch centers. The ratio of resistance to flow of the orificed member to the resistance to flow of the tip plate ranges from about 0.5 to about 1 (col. 1, lines 45-48).
U.S. Pat. No. 4,612,027 discloses a method and apparatus for forming glass fibers from a bushing having fields of "dripless" and "non-dripless" orifices, the "dripless" fields being positioned at the ends and corners of the bushing which are generally more prone to filament breaks (col. 3, lines 39-60). A pressure control plate is positioned within the bushing and has an outer region of high flow resistance to reduce the pressure at the orifices of the dripless fields and a central region of low flow resistance to maintain sufficient pressure at the orifices of the non-dripless fields to provide non-dripless operation (col. 3, line 67-col. 4, line 4). The dripless and non-dripless fields are isolated from each other by partitions extending between the pressure control plate and the tip plate (col. 4, lines 11-15).
U.S. Pat. No. 4,664,688 discloses a bushing including a pressure control plate having openings in its center and mechanical valves at opposed ends of the plate near the end walls of the bushing to permit variable control of the pressure of molten glass at the orifices of the tip plate (col. 3, lines 53-57). The flow resistance provided by the pressure control plate is increased when the valves are closed to render the bushing dripless during fiber forming. To facilitate restart of the fiber forming operation after fiber breakage, the valves are opened to decrease the flow resistance provided by the pressure control plate (col. 4., lines 6-16).
U.S. Pat. No. 3,988,135 discloses a bushing which includes a perforated reinforcing plate attached to the tip plate and a perforated deflector plate attached to the side walls of the bushing. The orifices of the reinforcing plate permit molten glass to pass through the reinforcing plate at a rate at least as great as the rate at which it passes through the orifices of the tip. plate (col. 4, lines 59-62). The deflector plate has a peaked construction to laterally deflect molten glass towards collection areas near the end walls of the bushing (col. 6, lines 24-29).
Russian Patent No. 562518 discloses a bushing assembly having a solid longitudinal divider and a screen filter positioned at the opposed ends of the bushing. The filter screens impurities from the melt.
The foregoing references do not recognize or address the effect of temperature variations in molten material located at the periphery versus the center of the bushing. Such variations can cause uneven filament diameters, disruption of the fiber attenuation process and accumulation of devitrified material in the corners of the bushing.